Method for varying valve timing of an internal combustion engine

ABSTRACT

A technique is used for adjusting the valve timing of an internal combustion engine with a cam shaft and a cam lobe displaced at a rotational angle with respect to one another. Under interposition of a cam following element, such as, for example, a bucket tappet, the cam shaft and the cam lobe work together with intake or exhaust valves as well as the camshaft actuator present on the camshaft, through which the position and/or the attitude of rotation of the camshaft to the crankshaft can be changed. To decrease the valve opening time, the camshaft can be adjusted toward hate” during a valve stroke, while the camshaft is adjusted toward “early” during the immediately subsequent base circle phase of the affected camshaft. To increase the valve opening times the camshaft can be adjusted toward “early” during a valve stroke, while the camshaft is adjusted toward “late” during the immediately subsequent base circle phase of the affected camshaft. This technique permits adjustment of the angular attitude of the camshaft as well influence on the valve stroke curve.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to a technique for changing the valve timing ofan internal combustion engine.

It is commonly known that the timing of an intake or exhaust valve of aninternal combustion engine can be changed by first changing the positionof the intake camshaft in relation to the exhaust camshaft by means of acamshaft actuator. The opening and closing times of the intake valvesare thereby adapted to a certain RPM ranges by two separate intaketimings and the valve overlap, while the valve opening duration and thevalve stroke remain the same (see, e.g. German document DE 38 76 762).Furthermore the prior art includes variable valve trains in which thevalve opening times as well as the valve opening widths are adapted tothe operating modes of the engines. German document DE 196 06 054 C2,for example, describes a variable valve train of an internal combustionengine in which the valve stroke can be adjusted between two settingswith the aid of a switchable bucket tappet. The assembly is equippedwith a device that adjusts the intake and exhaust times of the gasexchange valve, allowing improved filling of the cylinder over a largerange of RPMs.

It is an object of this invention to expand the functional range of acamshaft actuator in such a way that, in addition to adjusting the valvetiming by “shifting” the valve stroke curve toward intake or exhaust foran “early” or “late” opening, an adjustment of the valve opening timecan be achieved.

This object can be achieved through the claimed features.

For decreasing the valve opening time, the invention suggests adjustingthe camshaft in the direction of “late” during a valve stroke whileadjusting, that is correcting the camshaft in the direction of “early,”during the immediately subsequent base circle phase of the affectedcamshaft. To increase the valve opening time, which is determined by theshape of the cam lobe, the invention suggests adjusting the camshafttoward “early” during a valve stroke while adjusting, that is correctingthe camshaft in the direction of “late,” during the immediatelysubsequent base circle phase of the camshaft in question. By followingthese measures, the valve opening duration can be adjusted in anadvantageous manner. With the use of a camshaft actuator, a range offunction is thereby achieved, which is otherwise possible only with avariable valve train system.

Based on the features specified in dependent claims, additionaladvantageous configurations of the invention are also possible.

The technique for changing valve timing described is particularly suitedfor a double-rowed, 6-cylinder engine with a symmetrical ignitionsequence per cylinder bank, since, in engines of this type, an evendistribution of valve stroke and base circle phase on the camshaft arepresent.

The inventive procedure described is also suited for combination with avariable valve train, in which a valve stroke cross-over is present,because, as illustrated in the 2-setting valve cross-over exemplified inGerman document DE 196 06 054 C2, the base circle phase of the cam isespecially long with small valve strokes.

An embodiment of the invention is described in further detail in thefollowing description.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

The drawing FIGURE is a diagram showing the valve stroke on the intakeside of the internal combustion machine as well as the angular velocityof the camshaft.

DETAILED DESCRIPTION OF THE INVENTION

The inventive procedure is described based on a 6-cylinder boxer engine,both camshafts of which are equipped with camshaft actuator as well as atwo-setting valve stroke change-over on the intake side. Internalcombustion engines of this type are made by the Porsche Corporationunder the model series 996 “VarioCamPlus-System.” The construction andfunction of this variable valve train therefore does not need to bedescribed in further detail. With the aid of a hydraulically-activatedcamshaft actuator, the valve timing can be continuously adjusted on theintake side, while, because of the 2-setting valve cross-over, valvestroke curve A can be adjusted with a small cam lobe and valve strokecurve B can be adjusted with a large cam lobe as the diagramillustrates. The valve timing as well as the valve stroke are controlledas a function of the operating parameters of the internal combustionengine such as load or RPM. A possible field of the valve train in whichthe various operating modes of the valve train are plotted as a functionof RPM and load of the internal combustion engine is described andillustrated in German document DE 196 06 054 C2. As long as a continuousadjustment of the angle of rotation of the intake camshaft to theexhaust camshaft and to the crankshaft of the internal combustion engineis present, only valve stroke is shifted toward “early” or “late.” Theangular velocity of the camshaft is, up to the actual adjustmentprocess, constant, as shown by the dashed line C in the diagram.

Below the inventive procedure for decreasing or increasing the valveopening duration with the aid of a camshaft actuator is described infurther detail. The technique is particularly useful, when the internalcombustion engine is driven by small valve strokes, because the basecircle phase of the “small cam lobe” is particularly long.

1. Decreasing the Valve Stroke Curve.

During a valve stroke, the intake camshaft is adjusted toward “late” inthe amount Δφ, while the attitude of rotational angle of the camshaft isreset, that is, corrected toward “early” during the immediatelysubsequent base circle, preferably in the amount Δφ. This process isrepeated, provided that the valve opening duration for the currentoperating mode of the internal combustion engine is reduced relative tothe “normal” valve opening duration, which is determined by the valvecam lobes. The valve opening, curve D, illustrated in the diagram isthereby provided. Corresponding to valve opening, curve D, is theangular velocity of the camshaft, curve E, whereby it is evident that,during the valve stroke, a positive acceleration of the camshaft toward“late” occurs, while a negative acceleration of the camshaft toward“early” occurs due to the resetting during the base circle phase of theaffected cam lobe.

2. Increasing the Valve Stroke Curve.

In this case, the camshaft is adjusted toward “early” in the amount Δφduring the valve stroke, while the camshaft is corrected toward “late”during the base circle, preferably in the amount of Δφ.

The proposed procedure makes it possible to implement further valvestroke geometry with the same cam lobe shape. An influence of the valvestroke curve is then particularly important if a high adjustmentvelocity can be implemented with the camshaft actuator. An electricallycontrolled camshaft actuator of the prior art is particularlyadvantageous in this respect. The previously described procedure is, ashas already been mentioned, especially suited for double-rowed6-cylinder engines, without any restrictions to this configuration,because, in the case of the 1-6-2-4-3-5 ignition sequence used in thePorsche boxer engine, a symmetrical ignition sequence per cylinder bankcan be produced, thereby ensuring even distribution of valve stroke andbase circle phase on the camshaft. The procedure can also be used on theexhaust side of the internal combustion engine, wherein the exhaustcamshaft is also equipped with the necessary camshaft actuator in thiscase.

1-3. (canceled)
 4. A method for changing a valve timing of an internalcombustion engine with a camshaft and a cam lobe displaced at arotational angle with respect to each other, the camshaft and the camlobe, under interposition of at least one appropriate cam followingelement, acting together with intake or exhaust valves of the internalcombustion engine as well as with a camshaft actuator situated on thecamshaft, through which at least one of a position and a rotationalattitude of the camshaft can be adjusted for a crankshaft, comprising:(a) adjusting the camshaft toward “late” during a valve stroke andtoward “early” during an immediately subsequent base circle phase of thecamshaft to decrease a valve opening time, and (b) adjusting thecamshaft toward “early” during a valve stroke and toward “late” duringan immediately subsequent base circle phase of the camshaft to increasethe valve opening time.
 5. The method as claimed in claim 4, wherein theinternal combustion engine is a double-rowed, 6-cylinder engine withsymmetrical ignition sequence for each cylinder bank.
 6. The method asclaimed in claim 4, wherein the internal combustion engine is equippedwith a device for valve stroke change-over.
 7. The method as claimed inclaim 4, wherein the cam following element is a bucket tappet.
 8. Themethod as claimed in claim 5, wherein the internal combustion engine isequipped with a device for valve stroke change-over.
 9. The method asclaimed in claim 7, wherein the internal combustion engine is adouble-rowed, 6-cylinder engine with symmetrical ignition sequence foreach cylinder bank.
 10. The method as claimed in claim 7, wherein theinternal combustion engine is equipped with a device for valve strokechange-over.
 11. The method as claimed in claim 9, wherein the internalcombustion engine is equipped with a device for valve strokechange-over.